(1) Field of the Invention
The present invention concerns a device for connection or suspension between a detachable flexible or semi-flexible riser pipe and a pipe system on a fixed or floating platform or ship, which riser pipe is designed to transfer oil and/or gas and extends from a connection point on the sea bed to the platform or ship.
(2) State of the Prior Art
As oil and gas fields at great depths of the sea are developed, installations will increasingly be floating structures such as platforms, which move under the influence of waves, wind and currents. This means that the pipes which are linked to production and injection wells on the sea bed must be sufficiently flexible and strong to be able to move with the platform on the surface. For this purpose, special flexible riser pipes have been developed which withstand high pressure and have relatively high bending flexibility. As the movements and wave forces are greatest at the surface, the pipe is also exposed to the greatest bending stresses in this area. In particular, the stress will be great where the flexible pipe is connected to the platform, as this point is very stiff in relation to the pipe.
Various principles have been elaborated to limit the bending stresses at the suspension point of the pipe. One method which is widely used involves the use of a stiff guide pipe through the wave zone through which the flexible pipe passes. In order to limit the bending stress on the flexible pipe at the entrance to the guide pipe, the latter is provided with a trumpet-shaped end piece which controls the bending radius of the flexible pipe when it lies against the wall on the inside. In this way, the bending stress on the flexible pipe can be reduced by means of the shape of the trumpet-shaped end piece.
A more simple arrangement involves suspending the pipes freely from the deck. This produces high wave stresses on the pipes and it is particularly important to limit the resulting bending moments where the pipe is fastened to the deck. In such arrangements, it is common to use so-called bending limiters. These are specially manufactured flexible pipe sections which are placed outside the flexible pipe and which limit the bending radius of the flexible pipes with a precisely stepped bending stiffness. However, none of these solutions provide satisfactory security against overstressing, and thus the risk of breaking, when the bending stiffness of the pipe exceeds a certain magnitude.
Today, there are thus solutions for suspending "standard" flexible pipes from floating platforms. However, standard flexible pipes also have limitations regarding pressure and temperature, so that for some oil and gas fields it is very unfavourable or impossible to use this type of pipe. One possible solution is to use semi-flexible pipes. This means pipes which are made of metals with a low Young's modulus and a high tensile strength, such as titanium. These pipes will have acceptable stress tolerance in the area between the sea bed and the platform under the influence of waves and movement, but on account of the relatively high bending stiffness, it will be difficult or impossible to achieve an acceptable level of stress tolerance in the connection to the platform with the solutions described for standard flexible pipes.